Month: November 2010

I’ve been using a new HF antenna recently with surprisingly good results. Hopefully this page will be encouraging to those in apartments with severe antenna restrictions. I used to operate an indoor dipole mounted on my ceiling which was virtually invisible, but ever since solar panels were added to my apartment roof this antenna is picking up a huge amount of noise. In the past I played with a base-loaded vertical antenna made from copper pipe and it worked okay, especially when on my balcony, but it was bulkier than needed and awkward to store. My most recent antenna is made from 24AWG wire helically wrapped around the top element of a 3-element cane pole. My dad found a 15ft cane pole for $4 and it’s working pretty well for me. I guess the best description of this antenna is a “fully-loaded vertical” similar to a DIY hamstick. Here are some photos.

Notice that I only wrapped the highest element with wire. (The arrows in the above image depict where the helical element begins and finishes.) My logic is geared toward trying to get as much of the functional antenna above my apartment roof as possible. While it might not be a high-gain antenna, the level of noise reduction I experienced by raising the majority of the antenna above the roof is astounding. I can hear stations nearly full quieting that I cannot even detect with my indoor dipole. Also, I hate reports like this, but I’ve only made a few SSB contacts ever with my indoor setup, and always local US stations. The very first contact I made with this vertical antenna was Slovenia! He was calling CQ, I responded, and it picked me up on the first try.

THIS ANTENNA IS UGLY and a certain violation of my lease agreement which specifically states no outdoor antennas are allowed. Therefore, this is something I can only set up at night. Notice the PVC fitting at the base of the antenna – it makes it easy to set up and break down. Maximum setup / breakdown time is 30 seconds. On the floor of my balcony I have wire running up and down the wooden boards which forms a makeshift mesh ground plane. It’s not optimal, but I’m limited and it’s what I came up with. When I feel ambitious, I have quarter-wavelength radials that I toss off the balcony and in the bushes to improve grounding. Although I’m sure I could have tap points and gator clips to select the antenna’s resonant frequency, currently I run the antenna right into a MAC-200 antenna tuner. I’ve used it on 17m, 20m, 30m, 40m, and 80m. Again, this antenna is far from optimal, it should represent the last resort for extreme cases, but when you’re faced with not being able to operate at all this little quick and dirty setup has been a godsend!

I’m sure a lot of people will read this and be angry or argue as to why this doesn’t make a good antenna. I’m not claiming it’s awesome, but for me it’s the best I could come up with in my limited situation. That’s my $0.02!

Now that I’ve finished my 6-channel data logger (previous post), it’s time to put it to the test! I’m using a handful of LM335 temperature sensors to measure temperature, and a 20 Ohm resistor to act as a heater. When 1A of current passes through it, it gets quite toasty! First, I’ll make some temperature probes…

UPDATE: Those photos show a partially completed sensor. Obviously the third wire is required between the resistor and the LM335 to allow for measurement! Here’s a more completed sensor before the shrink tube was massaged over the electrical elements:

Then I mounted the sensors on a block of steel with the heater on one side. This way I can use one temperature to measure the heater temperature, and the other to measure the temperature of the metal chassis. I then put the whole thing in a small Styrofoam box.

When I fire the heater, that sucker gets pretty darn hot. In 40 minutes it got almost 250F (!) at which time I pulled the plug on the heater and watched the whole thing cool. Notice how the metal chassis lags behind the temperature of the heater. I guess it’s a bit of a “thermal low-pass filter”. Also, yes, I’m aware I spelled chassis incorrectly in the graphs.

But how do we use this to build a thermo-stable crystal oven for a MEPT (radio transmitter)? I tried a lot of code, simply “if it’s too cold, turn heater on / if it’s too hot, turn heater off” but because the chassis always swung behind the heater, and even the heater itself had a bit of a delay in heating up, the results were always slowly oscillating temperatures around 10F every 20 min. That’s worse than no heater! My best luck was a program to hold temperature stable at 100F with the following rules:1.) If heater > 155F, turn heater off (prevent fire)2.) If chassis < 100F, turn heater on3.) if (heater-target) > (target-chassis), turn heater off

What a great job! That thing is practically stable in 20 minutes. The advantage of this over an analog method is that I can set the temperature in software (or provide an interface to change temperature) and my readings are analytical, such that they can be conveyed in a radio message. Again, my best results came when I implemented rule 3 in the code above. More experiments to come!

While working to perfect my temperature-controlled manned experimental propagation transmitter (MEPT), I developed the need to accurately measure temperature inside my Styrofoam enclosure (to assess drift) and compare it to external temperature (to assess insulation effects). I accomplished this utilizing the 8 ADC channels of the ATMega48 and used its in-chip USART capabilities to send this data to a PC for logging. I chose the ATMega48 over the ATTiny2313 (which has USART but no ADCs) and the ATTiny44a (which has ADCs but no USART). From when I see, no ATTiny series ATMEL AVR has both! Lucky for me, the ATMega48 is cheap at $2.84 USD. Here’s my basic circuit idea:

EDIT: the voltage reference diagram is wrong at the bottom involving the zener diode. Reference the picture to the right for the CORRECT way to use such a diode as a voltage reference. (stupid me!)

MULTIPLE SENSORS – Although in this demonstration post I only show a single sensor, it’s possible to easily have 8 sensors in use simultaneously since the ATMega48 has 8 ADC pins, and even more (infinitely) if you want to design a clever way to switch between them.

LM335 Temperature Sensor – selected because it’s pretty cheap (< $1) and quantitative. In other words, every 10mV drop in voltage corresponds to a change of 1ºC. If I wanted to be even cheaper, I would use thermistors (zener diode (perhaps 4.1V?) as a voltage reference.

Here is my circuit. I’m clocking the chip at 9.21MHz which works well for 19200 baud for serial communication. Refer to my other MAX232 posts for a more detailed explanation of how I chose this value. The temperature sensor (blurry) is toward the camera, and the max232 is near the back. Is that an eyelash on the right? Gross!

The data is read by a Python script which watches the serial port for data and averages 10 ADC values together to produce a value with one more significant digit. This was my way of overcoming continuously-fluctuating values.

Here you can see me testing the device by placing an ice cube on the temperature sensor. I had to be careful to try to avoid getting water in the electrical connections. I noticed that when I pressed the ice against the sensor firmly, it cooled at a rate different than if I simply left the ice near it.

NOTICE THE PROGRAMMER in the background (slightly blurry). The orange wires connect the AVR programmer to my circuit, and after the final code is completed and loaded onto the microcontroller these orange wires will be cut away.

Here is some actual data from the device. The LM335 readout is in Kelvin, such that 3.00V implies 300K = 80ºF = 27ºC (room temperature). The data is smooth until I touch it with the soldering iron (spike), then it gets cool again and I touch it with a cold piece of metal (wimpy dip), then later I put an ice cube on it (bigger dip). Pretty good huh? Remember, 0.01V change = 1ºC change. The bottom of the dip is about 2.8V = 280K = 44ºF = 7ºC. If I left the cube on longer, I imagine it would reach 0ºC (273K, or 2.73V).

UPDATE: A day later I added multiple sensors to the device. I calibrated one of them by putting it in a plastic bag and letting it set in ice water, then I calibrated the rest to that one. You can see as my room temperature slowly falls for the night, the open air sensor (red) drops faster than the insulated one in a Styrofoam box. Also, I did a touch of math to convert voltage to kelvin to Fahrenheit. You can also see spikes where it quickly approached 90+ degrees from the heat of my fingers as I handled the sensor. Cool!

UPDATE: a day and a half later, here’s what the fluctuations look like. Notice the cooling of night, the heating of day, and now (near the end of the graph) the scattered rain causes more rapid fluctuations. Also, although one sensor is in an insulated styrofoam box, it still fluctuates considerably. This measurement system is prepped and ready to go for crystal oven tests!

I’ve been pretty busy lately, but I drip to the hardware store with the XYL produced a PVC enclosure that looked perfect for my ongoing MEPT (manned experimental propagation transmitter) projects. I didn’t want to buy it (it was a little pricey by my standards, at $6 USD, which is about the total cost of the transmitter!) but the wife convinced me and I’m glad she did! I intended it to replace the styrofoam enclosure I had been using, but I wasn’t thinking clearly and drilled holes in the box and mounted screws through them. While electrically this is a wonderful way to add antenna connections, thermally it was a bad idea. The main point of the enclosure was to be temperature stable! Oh well. I put the whole thing in the Styrofoam and as a test, I’m leaving it outside tonight. I can’t wait to see how it goes! Here are some photos of the project.

Update: Even when being housed outdoors when temperature fluctuations vary greatly between day and night, this MEPT is surprisingly stable! When I open the box, it’s very warm inside, so I am thinking that the voltage regulators and the MOSTFETs of the PA are heating the device nicely. Here’s a capture spanning about 2 hours. The vertical height of each “V” is about 10Hz, so I estimate that for this span of time, drift is <1Hz. However, I do believe that long term (day to day) frequency stability is still not optimal, but only time will tell.

Signal report: briefly, this is my signal in Alaska courtesy of KL7UK. My signal is the V-shaped one near the bottom:

Although I’ve been ridiculously busy the last few weeks, I’ve been eying some posts circulating around the Knights QRSS mailing list regarding mysterious signals in the 40m band. They recognized it as a QR Code and tried decoding it with phones and such, but the signal wasn’t strong enough above the noise to be automatically deciphered.

That’s the original spectrograph as captured by ON5EX in Belgium. It’s a pretty good capture, it’s just not great enough to be automatically decoded. The first thing I did was pop it open in ImageJ, separate the channels, and use the most useful one (red, I believe). When adjusted for brightness and contrast, I was already at a pretty good starting point.

I tried using an automated decoder at this point (http://zxing.org/w/decode.jspx) but it wasn’t able to recognize the QR code. I don’t blame it! It was pretty rough. I decided to manually recreate one, so I slapped the image into InkScape, add a grid, and align the image with the grid. From there, it was as easy as drawing a single grid-square-sized rectangle and copy/pasting it in all the right places.

However problems came when working on those last few rows. The static was pretty serious, so I tried a lot of different filters / adjustments. One of the greatest benefits was when I stretched the image super-wide and performed a “rolling ball” background subtraction, then revered it to its original size. That greatly helped reduce the effect of the vertical striping, and let me visually determine where to place the last few squares by squinting a bit.

Once it was all done, I saved the output as orange, then later converted it to black and white for web-detection via the ZXing Decoder.

The final result:

… which when decoded reads:

WELL DONE / F4GKA QSL PSE 73

Yay! I did it. Although my call sign is AJ4VD, I’m spending the afternoon at the University of Florida Gator Amateur Radio Club station and am using their computers, so I might QSL as W4DFU. Also, there’s a lot to be said for ON5EX for capturing/reporting the QR code in the first place. After a bit of research, it turns out that F4GKA is one of the Knights! I should have known it =o)

About Scott

Scott Harden lives in Gainesville, Florida and works at the University of Florida as a biological research scientist studying cellular neurophysiology. Scott has lifelong passion for computer programming and electrical engineering, and in his spare time enjoys building small electrical devices and writing cross-platform open-source software. more →